Importance of Individual Species of Predators on Nesting Success of Ducks
in the Canadian Prairie Pothole Region

Results

Analysis of the explanatory variables
Of the five carnivores, the coyote had the highest average index of activity (Table
1), followed by the skunk, fox, raccoon, and badger. Indices for magpies were
much lower than those for crows, and the Franklin's ground squirrel had a very
low index. These results do not necessarily indicate the relative abundance of
the different predators; indices are of three non-comparable types and, even for
the five carnivores for which indices were based on tracks, equal values need
not translate to equal abundance. The Franklin's ground squirrel and magpie seemed
the most variable, as measured by the coefficient of variation. The coyote was
the least variable, followed by the skunk.

Table 1. Explanatory
variable: mean, interquartile range (IQR), and coefficient of variation
(CV) for indices of activity of eight predators, indices of two wetland
types, and a measure of spring precipitation

Mean

IQR

CV (%)

Badger

0.25

0-0.40

142

Raccoon

0.36

0-0.53

162

Striped skunk

1.01

0.42-1.54

69

Red fox

0.73

0-1.26

109

Coyote

1.56

0.84-2.23

56

Crow

0.45

0.08-0.63

114

Magpie

0.022

0-0.027

190

Franklin's ground squirrel

0.011

0-0.008

232

Seasonal wetlands

58.3

37-86

51

Semipermanent wetlands

85.5

78-98

20

Precipitation (cm)

13.4

10.7-16.5

32

Several highly significant correlations existed among the predator indices
(Table 2). The strongest was the negative relationship between fox and coyote
indices (r = -0.51), a finding consistent with the spatial avoidance
and agonistic behavior between these two canids (Dekker 1983; Voigt and Earle
1983; Sargeant et al. 1987a; Major and Sherburne 1987). Also, indices
of the badger and skunk were negatively related (r = -0.46). Little is
known of relations between these two species, but Sargeant et al. (1982) reported
two instances of badgers preying on young skunks. Two other patterns were evident:
(i) a general association among indices of the fox, skunk, and raccoon;
and (ii) an association among indices of coyote, magpie, and Franklin's
ground squirrel. Crow indices were positively related to those of the Franklin's
ground squirrel, skunk, magpie, and coyote, and negatively related to raccoon
and badger. Raccoons climb trees and may depredate crow nests. Badger indices
were positively associated with those of no other species, but negatively associated
with indices of the skunk, magpie, and crow.

Table
2. Correlation coefficients between explanatory variables

Raccoon

Skunk

Fox

Coyote

Crow

Magpie

Ground
squirrel

Seasonal

Semipermanent

Precipitation

Badger

0.06

-0.46

-0.08

0.04

-0.11

-0.15

0.02

-0.06

-0.01

-0.19

Raccoon

0.16

0.22

-0.11

-0.19

-0.04

-0.05

0.21

0.16

-0.05

Striped skunk

0.34

-0.01

0.16

-0.06

-0.02

0.04

-0.03

0.15

Red fox

-0.51

-0.05

-0.15

-0.18

0.18

0.08

-0.07

Coyote

0.12

0.15

0.15

-0.17

-0.03

0.35

American crow

0.14

0.32

-0.09

-0.17

-0.03

Black-billed magpie

0.08

-0.16

-0.12

0.06

Franklin's ground squirrel

0.05

-0.04

-0.00

Seasonal wetlands

0.78

0.35

Semipermanent wetlands

0.49

NOTE: Significance
levels for |r| are r0.05
= 0.06 and r0.01 = 0.08.

Correlations with seasonal wetland counts were positive for the raccoon and
fox, and negative for the coyote and magpie (Table 2). Correlations with semipermanent
wetland counts were positive for raccoon and negative for crow and magpie. Correlations
with precipitation indices were positive for coyote and skunk and negative for
badger. These correlations between predator indices and pond counts or precipitation
indices represent association; we mean to imply no cause-and-effect relationship.
The precipitation index was positively correlated with indices of both seasonal
(r = 0.35) and semipermanent (r = 0.49) wetlands. The correlation
between the two wetland indices was 0.78.

Analysis of daily predation rate on early nests
There were 559 observations (quarters with nests found during the early searches)
for this analysis. Included were 1791 nests, mostly mallards (Anas platyrhynchos),
blue-winged teal (Anas discors), northern pintails (Anas acuta),
and northern shovelers (Anas spatula) (Table 3). The average daily predation
rate on early nests was 0.057, with an interquartile range of 0.033-0.158. Correlations
between daily predation rate of early nests and the explanatory variables, weighted
by exposure, were positive for fox, crow, and skunk, and negative for coyote
and both wetland counts (Table 4). The analysis of variance for the full model
was significant overall (F = 3.84, df = 11,547, P < 0.0001). Standardized
regression coefficients are given in Table 4; these were obtained by subtracting
from each explanatory variable its mean and dividing by its standard deviation.
Regression coefficients were positive for indices of all predators except coyote
and magpie, but because of intercorrelations among the explanatory variables,
these coefficients were not all well-conditioned and are difficult to interpret.
We successively eliminated nonsignificant explanatory variables, and ultimately
obtained an equation involving only indices of fox, crow, badger, and the seasonal
wetland count (Table 4). Coefficients of the three predator indices were positive
and that of the wetland count was negative. The overall equation was highly
significant (F = 8.77, df = 4,554, P < 0.0001) despite a small
multiple correlation coefficient (R2
= 0.060). The badger index was marginally significant in the final equation,
in spite of its minor importance in the initial one. Conversely, the skunk index
was absent in the final equation, although it correlated rather strongly with
daily predation rate. These discrepancies resulted from the intercorrelations
among the explanatory variables; for example, the pronounced correlation between
fox and skunk indices (r = 0.34) made it difficult to distinguish the
separate effects of the two species.

Table 3. Species composition
of early and late clutches used in the analysis

Early clutches

Late clutches

Mallard (Anas platyrhynchos)

735 (41.8%)

294 (22.2%)

Gadwall (Anas strepera)

64 (3.6%)

288 (21.8%)

American wigeon (Anas penelope)

43 (2.4%)

88 (6.7%)

Green-winged teal (Anas crecca)

21 (1.2%)

23 (1.7%)

Blue-winged teal (Anas discors)

371 (20.7%)

308 (23.3%)

Northern shoveler (Anas spatula)

195 (10.9%)

139 (10.5%)

Northern pintail (Anas acuta)

352 (19.7%)

105 (7.9%)

Lesser scaup (Aythya affinis)

6 (0.3%)

76 (5.7%)

Other

4 (0.2%)

2 (0.2%)

Total

1791

1323

Table 4. Correlation
coefficients and standardized regression coefficients relating daily predation
rates of early and late clutches to explanatory variables

Early
clutches

Late
clutches

Correlation

Regression
coefficients

Correlation

Regression
coefficients

Initial

Final

Initial

Final

Badger

0.03

0.038

0.027*

-0.04

-0.011

Raccoon

0.01

0.019

0.02

0.006

Striped skunk

0.09

0.010

0.16

0.028

0.027

Red fox

0.18

0.048

0.069***

0.21

0.052

0.055**

Coyote

-0.09

-0.042

-0.01

0

American crow

0.11

0.037

0.039**

0.06

0.006

Black-billed magpie

-0.02

0.005

0.04

0.004

Franklin's ground squirrel

0.04

0.013

-0.02

0.000

Seasonal wetlands

-0.08

-0.034

-0.037**

-0.10

0.033

Semipermanent wetlands

-0.11

-0.028

-0.18

-0.087

-0.060***

Precipitation

-0.01

0.037

0.02

0.001

*P < 0.10.
**P < 0.01.
***P < 0.0001.

Analysis of daily predation rate on late nests
We had 424 observations available for this analysis, involving 1323 nests of
mostly blue-winged teal, mallards, and gadwalls (Anas strepera) (Table
3). The average daily predation rate was 0.044, with an interquartile range
of 0-0.146. Correlations between daily predation rate of late nests and the
explanatory variables, weighted by exposure, were positive for fox and skunk,
and negative for the wetland indices, especially that of semipermanent wetlands
(Table 4). The analysis of variance for the full model was significant (F
= 3.69, df = 11,412, P < 0.0001). Regression coefficients are given
in Table 4. Regression coefficients for predator indices were positive except
for those of badger and Franklin's ground squirrel. After successively eliminating
nonsignificant explanatory variables, we obtained an equation involving only
indices of fox, skunk, and the semipermanent wetland count (Table 4). Regression
coefficients of both predator indices were positive, and that of the wetland
count was negative. The overall equation was highly significant (F =
12.98, df = 3,420, P < 0.0001), with R2
= 0.085. The skunk index was retained in this equation despite its marginal
significance (P = 0.11) because of its strong first-order correlation
with daily predation rate (Table 4).